Composition and structure of triacylglycerols in brown adipose tissue of rats fed fish oil

Abstract This study examines the incorporation of highly unsaturated n−3 fatty acids (HUFA) into triacylglycerols (TAG) of brown adipose tissue (BAT), and their effect on the positional distribution of saturated (SFA) and of unsaturated (UFA) 16‐ or 18‐carbon fatty acids. To this end, rats were fed a fish oil diet for up to four weeks. The stereospecific analysis of TAG was based on generation of sn ‐1,2‐ and sn ‐2,3‐acylglycerols by Grignard degradation, followed by synthesis of phosphatidic acid and specific hydrolysis with phospholipase A 2 . From the end of the first week of fish oil feeding, a steady‐state in the fatty acid composition of TAG in BAT was reached. HUFA concentration increased 30‐fold, mainly at the expense of n−9 UFA and of SFA. The amount of SFA decreased selectively at position 3, where these fatty acids were progressively replaced by n−3 HUFA. By contrast, the amount of UFA decreased at all positions, and their positional distribution was not affected. About 60% of HUFA was incorporated at position 3. Nearly twice as much 22∶6n−3 was incorporated into TAG than had been previously observed in white adipose tissue (WAT) [Leray, C., Raclot, T., and Groscolas, R. (1993) Lipids 28 , 279–284]. At the steady‐state, the distribution of HUFA was characterized by high proportions of 22∶6n−3 and 20∶5n−3 in position 3. Moreover, in each position of TAG, a steady level was reached rapidly (within 1 wk). It is concluded that, during fish‐oil feeding, fatty acids in TAG of BAT show characteristic time‐course changes that lead to a characteristic composition and a tissue‐specific positional distribution. This suggests that adipose tissue has its own specificity in controlling the build‐up of TAG stores, which is likely to be regulated by the specificity of acylating enzymes as well as molecular rearrangements.